In U.S. Pat. No. 8,212,283 B2 a prior art reverse-conducting insulated gate bipolar transistor (RC-IGBT) in form of a Bi-mode Insulated Gate Transistor (BIGT) 160 is described (shown in FIG. 1), which comprises a freewheeling diode and an insulated gate bipolar transistor (IGBT) on a common semiconductor chip, part of which chip forms the (n−) doped drift layer 5 with a drift layer doping concentration and a drift layer thickness 53. The RC-IGBT comprises an anode side 27 (second main side) and a cathode side 20 (first main side), whereas the anode side 27 is arranged opposite of the cathode side 20 of the chip.
The drift layer thickness 53 is the maximum vertical distance between the anode and cathode side 27 and 20 of that part of the chip with the drift layer doping concentration.
An n doped source region 3, a p doped base layer 4 and a gate electrode 6 having an electrically conductive gate layer 62 and an insulating layer (comprising a first and second insulating layer 64, 66), which insulates the gate layer 62 from any doped layer, and a cathode electrode 2 (first main electrode), are arranged at the cathode side 20.
The reverse-conducting semiconductor device comprises an active cell region 10, which is an area in the chip, which includes and is arranged below any of the source region 3, base layer 4 or gate layer 62.
In such a BIGT, an n doped first layer 50 of higher doping concentration than the drift layer doping concentration of the drift layer and a p doped anode layer 55 are alternately arranged on the anode side 27. The first layer 50 comprises at least one or a plurality of n doped first regions 51, wherein each first region has a first region width 52. Each first region width 52 is smaller than the drift layer thickness 53.
The anode layer 55 comprises at least one or a plurality of p doped second regions 56 and at least one or a plurality of p doped pilot regions 58, wherein each second region 56 has a second region width 57 and each pilot region 58 has a pilot region width 59 (FIG. 3). The at least one second region 56 is that part of the anode layer 55, which is not the at least one pilot region 58 (which will be defined in the following).
A mixed region comprises the at least one first and second regions 51, 56. The mixed region is arranged between the pilot region 58 and the active region border and has a width of at least once the base layer thickness 53. FIG. 2 shows a cut through the line A′-A′ of FIG. 1 showing such a mixed region.
Any region (first, second or pilot region) has a region width and a region area, which is surrounded by a region border. A shortest distance shall be the minimum length between a point within said region area and a point on said region border. Each region width is defined as two times the maximum value of all (i.e. any possible) shortest distances within said region.
Each pilot region area is a p doped area, in which any two first regions 51, which are arranged on the border of the pilot region 58, have a distance between two neighboured first regions 51 on the pilot region border smaller than two times the drift layer thickness 53. The pilot region width 59 is at least two times the drift layer thickness 53. In an exemplary embodiment, the drift layer thickness 53 is at least 100 μm (for a device of about 1200 V), at least 300 μm (for a device of about 2500 V) and at least 500 μm (for a device of about 4500 V). Thus, the pilot region width is at least 200 μm, at least 600 μm or at least 1000 μm. The total area of the at least one pilot region 58 is between 10% and 30% of the area of the active region.
The pilot region 58 is laterally surrounded on the pilot region border by first regions 51, which have a distance from each other of less than two times the drift layer thickness 53, exemplarily smaller than once the drift layer thickness 53. No n doped region 51 is enclosed in this p doped pilot region 58. That means that the at least one first region 51 surrounds the at least one pilot region 58 in a plane parallel to the anode side 27 such that an n doped area (i.e. first region(s)), which has at least one opening (i.e. in which p doped second regions 56 are arranged) of less than two times the drift layer thickness 53 or which has no such openings, surrounds the at least one pilot region 58. This shall include the option that the first region 51 is a continuous region surrounding the pilot region 58 in a plane parallel to the anode side 27 or by having a first region 51 formed as an open ring having an opening smaller than two times the drift layer thickness 53. By having an n doped area around the pilot region 58, p doped areas have a width of less than two times the drift layer thickness 53 (and thus form second regions 56) are arranged.
In the pilot region 58, no first region is arranged or enclosed. Across the pilot region 58 (i.e. on the pilot region border), the first regions 51 have a distance of more than two times the drift layer thickness 53. That means that the pilot region 58 may be enclosed by first regions 51 which have a smaller distance to each other, but across the pilot region area, the distance between any two first regions 51 is larger than two times the drift layer thickness 53. In other exemplary embodiments, each pilot region area has a width larger than 2.5, in particular 3 times or 4 times the drift layer thickness 53.
The pilot region has a pilot region area such that a circle (p doped area) having a diameter of at least two times the drift layer thickness 53 can be laid into the pilot region over the whole region area in a plane parallel to the anode side 27.
The at least one pilot region 58 is arranged in the central part of the active region 10 in such a way that there is a minimum distance between the pilot region border to the active region border 580 of at least one time the drift layer thickness 53 (FIG. 3). The pilot region 58 represents a pilot IGBT region, which is surrounded by shorted regions with alternating first and second doped regions 51, 56 (mixed region). The at least one pilot region 58 is arranged in the central part of the device such that the mixed region laterally surrounds the at least one pilot region 58.
By the introduction of the pilot region 58 with much increased dimensions compared to the first and second regions 51, 56, a region is created which is dedicated as sole IGBT region and not operating in the diode mode. The p-type pilot region 58 ensures snap-back free operation of the BIGT. The pilot region 58 can also be used to give more freedom to determine the IGBT to diode area ratio and decouple this design aspect from the standard approach involving the small second regions 56 only.
The pilot region 58 represents a pilot IGBT region, which eliminates snap-back effects at low currents. The snap-back effect of a BIGT depends on the resistance of the drift layer, which in turn depends on the resistivity and thickness of the drift layer 53. For devices having a greater drift layer thickness 53, the voltage drop across the drift layer is larger. Therefore, also the total on-state voltage drop is higher for such devices, and snap-back effect occurs at higher voltages.
The introduction of a sufficiently large p doped region (pilot region) can avoid such snap-back effect in a high voltage IGBT device. A minimum distance between this pilot region 58 and the border of the active region is essential for good thermal performance and improvement of the device SOA since the pilot IGBT does not include transition parts of the chip such as those from active to termination regions. Furthermore, by using a pilot region 58, snap-back behaviour is improved compared to distributed smaller pilot regions.
The first and second regions 51, 56 form the main shorted region in which the silicon area included is utilized in both IGBT and diode mode.
The n doped anode shorts (first layer) conduct electron current during the turn-off and give rise to the FCE effect which greatly improves turn-off softness of the BIGT device.
In a prior art reverse conducting (RC)-IGBT or BIGT device the p-base layers of the MOS cells are utilized as anode regions of the internal freewheeling PIN diode. The MOS cells are terminated at the termination region with deeper and higher doped p-bars, which also act as additional diode anode regions. These additional p-bars are not shorted by the MOS channel and are therefore essential for achieving snap-back free diode mode characteristics, as is described in the patent application US 2013/0099279 A1. However, the SOA of the freewheeling diode is affected strongly by the design of the p-bars contact with the termination region in the areas around the active area of the BIGT/RC-IGBT.
The standard design of the RC IGBT and BIGT uses the additional p-bars which are diffused deeper and are higher doped compared to the p-base layer of the MOS cell. They contact the p base layer in an area, in which no MOS channel is formable.
When the freewheeling diode is conducting, the p-bars and termination act as anode regions and inject holes into the n-drift layer. During reverse recovery the electrical field maximum is near the deep and highly doped p-bars, and as a result the reverse current becomes concentrated in these areas. Thus, a current filament, in which holes are highly concentrated in a small area, is created at the curvature of the p-bar. As a result of this, a locally high temperature is created in the filament which might lead to device destruction and therefore lowers the diode mode SOA as shown by Prior art curve in the thermodynamic simulation (FIG. 4).
Chen shows in “A snapback suppressed RC-IGBT with built-in diode by utilizing edge termination” Superlattices and Microstructures, vol. 70, 109-116 a prior art RC-IGBT having in a central region a pure IGBT region. This IGBT region is surrounded by a pure diode region, which is established between a p doped bar, which is electrically connected to the cathode electrode, and an n doped layer on the second main side. In a termination region surrounding the diode region, field ring are arranged, which have the same depth as the p bar. These field ring structure has a width of 280 μm, which great width is needed for the reduction of the electric field.
WO 2014/054319 A1 shows an IGBT, which comprises on the first main side p doped diffused regions, which have increased distance to each other with increasing distance from the active cell region. The inner p regions close to the active cell region are connected to the cathode electrode, whereas in the outer region the p regions are separated from each other by the drift layer, so that they function as field rings.
For terminating an electric field with such isolated field ring structures, a larger number of field rings are needed, which together with the distance between the field rings results in a wide termination region.